Transformer bank



March 15, 1960 J. KREUZER TRANSFORMER BANK 3 Sheets-Sheet 1 Filed June 10, 1957 3 Sheets-Sheet 2 Filed June 10, 1957 March 15, 1960 J. KREUZER 2,929,016

TRANSFORMER BANK Filed June 10, 1957 3 Sheets-Sheet I5 United States Patent 2,929,016 1 TRANSFORMER BANK Josef Kreuzer, Graf, Styria, Austria, assignor to Elin Aktiengesellschaft fiirEle'ktrische Industrie, Vienna, Austria, a corporation of Austria Application time 10, 1957, serial No. 671,369 7 Claims. c1. 323 44 This invention relates to a transformer bank and in particular to circuits connecting the transformer bank to generators.

in recent practice large three-phase transformers have been replaced by three single-phase units. There. are various reasons for this solution. One of these reasons is the fact that the disposal of reserve units is more economical. The increased expenses due to the associated com plication of the required switches, is, in most cases, more than compensated by this arrangement. Another reason is the possibility of incorporating the load regulating devices into the single-phase units. In the former case of a three-phase transformer the regulation requires the installation of a separate regulating transformer. Agreement as to the transformercapacity above which the construction of a bank is preferable, has not been achieved as yet. For very high capacitiesand very high voltages,- as for example 380- kv., the bank indicated by (1) corn s'truction features: above all the necessary clearances between the single transformer legs and between the bush ing insulators result in the construction of three-phase transformers which exceed the dimensions admissible in railway transportation; (2) electrical considerations: the short-circuit capacity on the generator side can be con trolled by commercial circuit breakers.

In such cases, it is well-known practice tosum up the capacity of two threephase generators ina transformer bank that consists of three two-legged single-phase transformers, each leg being provided with a low voltage winding (primary windings) fed bythe two generators, and a high voltage winding (secondary windings) connected in parallel. If the bank is supplied by three generators, their capacities are totalized, in accordance with common usage, in three five-legged transformers only three legs of which are wound; Each leg is supplied by one generator phase. The high voltage windings of the three legs of a single-phase unit are connected in parallel. Uniform distribution ofload between two and three generators resp; is ensured by the fact that due to the geometrical identity of the winding arrangement the reactance and, therefore, the short-circuit voltages between a low voltage winding and the corresponding high voltage winding on the same leg are the same. The principal advantage is thatthe arrangement ofthe primary windings on different legs allows the reactance betweentwo primary windings to equal about double the re'actance between the primary winding and the secondary winding. Therefore, in case of a short circuit, the generators are well protected and the short circuit breaking capacity on the generator side is divided so that adequate control of such breaking capacities can be achieved by commercial circuit breakers. Besides, this connection permits to operate any generator independently of the other generators.

The present invention suggests the use of specially wound two-legged single-phase transformers in such a transformer bank. Two primary windings and one secondary winding are provided on each leg of each of the two-legged transformers; the secondary winding may be r; 2,929,016 Ice Patented Mar. 15,1950

split. All windings extend over substantially the total length of the leg. Further, one of the primary windings on each leg is dimensioned for full phase capacity, while the other winding is dimensioned for half-phase capacity, the two half-phase capacity windings being electrically connected. At least a portion of the secondary winding is positioned in between the two primary windings. It will be appreciated that the use of two-legged single-phase transformers in transformer banks for three three phase generators afiords a considerable advantage. This advan tage is secured by the particular arrangement and dime'ni sioning of the primary and secondary windings" on the transformer legs. 4 p w This invention will be better understood from" the fol lowing description taken in connection with the accom-" panying drawings, in which,

Fig. l is a schematic circuit diagram of three threephase generators supplying three single-phase transformers in a transformer bank according to one embodiment or the invention;

Fig. 2 shows the designations of the geometrical di-" mensions used in the formulas presented in columns2 and 3 of the specification; U I Fig. 3 is a schematic circuit diagram showing one single-phase transformer wound and connected according to one special feature of the present invention; and v Fig. 4 is a schematic circuit diagram showing a three: phase generator supplying three single-phase transformers in a transformer bank according to another embodiment of the invention. W

In the transformer bank T T T shown in Fig. 1 each of the six transformer legs carries a high voltage winding H and two low voltage windings either N N or N N The high voltage windings H of both legs of each transformer are connected in parallel. 1hiv voltage windings N N of the transformer T dim'eii sioned for full-phase capacity, are assigned to phase it of the first or the second generator'G or G respective and the low voltage windings N and N dimensioned for half-phase capacity, are connected in series to the phase a of the third generator G The legs of the two other transformers T and T of the bank, assigned to phases v and w, are wound and connected in an untsens way as shown. In order to ensure a uniform distriliu tion of the primary capacity (in particular the short circuit capacity) on the three generators, it is necessary to have the same percentage short-circuit voltage up on all three primary windings. As is generally known'these short-circuit voltages are given by the following ex-f pression. (Fig. 2 illustrates the dimensions involved in the following expressions as well as the definitionsof 1: S2: S3 3 v -a u 1.24 u 4h 0 10 (for. full phase capacity) D3IS3I D3IIS3II D3S3 we find as condition for an equal distribution of load:

kD1S =kD2S =k /2D3S3 In order to satisfy this equation, the value of the product D s must be twice the value of the product D s and D s respectively, i.e. the leakage gaps between H and N and between H and N must be considerably larger than the gaps between H and N and between H and N respectively. For obvious reasons the width of any leakage gap, generally, will not be chosen larger than the minimum required for electrical reasons (clearance). Therefore, over-dimensioning the leakage gaps between H and N and between H and N according to the above equation of condition is, in many cases, a disadvantage.

If the high voltage winding is solidly grounded, as is the case for transformer capacities so large as to require replacement of the three-phase transformer by a bank, the above disadvantage can be avoided by winding a small part of the high voltage winding directly on the core. The first low voltage winding for full-phase capacity, the remaining high voltage winding and the second low voltage winding for half-phase capacity are arranged in this sequence from inside to outside. This arrangement decreases the short-circuit voltages, u and a and, simultaneously, increases the short-circuit voltage a Thus, a good approximation of the desired short-circuit voltages with about the same size leakage gaps can be obtained by arranging a relatively small part of the secondary winding directly on the core. Alternatively, the two primary windings N and N may be connected in parallel.

' Fig. 3 shows a single-phase unit the windings of which are distributed on both legs in the above mentioned manner. This distribution affords uniform splitting of the load using leakage gaps of the same dimensions, whereas the sizes of the leakage gaps are determined primarily by electrical reasons. H and I indicate the two sections of the high voltage winding on the same leg connected in series, N and N the low voltage windings for full-phase capacity, each wound on one leg, and N and N the low-voltage windings for half-phase capacity, each wound on one leg, and connected in series.

The circuit shown in Fig. 4 is similar to the one shown in Fig. 1 and the identical connections need therefore not be explained in detail. However, the low-voltage windings N and N dimensioned for half-phase capacity, of each transformer leg are connected in parallel in the embodiment of Fig. 4; these windings are connected in series in the embodiment of Fig. 1.

A single-phase unit using the arrangement of windings according to the present invention, is not markedly wider but substantially shorter than a transformer unit with a five-legged core. Weight and iron losses of a two-legged core are considerably smaller than those of a five-legged core. With the double concentric winding arrangement decreasing the additional copper losses, the winding arrangement in accordance with the present invention and the resulting load distribution allow not only a reduction of weight and, therefore, of cost but also an improvement in efficiency. With regard to load distribution and generator protection against short circuits, the disclosed winding arrangement affords the same advantages as the wellknown constructions such as: the same short-circuit voltages between the three primary windings and the high voltage winding and, approximately double the impedance from one generator winding to the next one.

If desired, the high voltage winding can be connected as an autotransformer in order to transform the voltage supplied by the three generators into two different voltages to supply two separate networks of different voltages.

What 1 claim as new and desire to secure by Letters Patent of the United States is:

1. In a bank of three single-phase transformers supplied by three three-phase generators, a two-legged transformer, two first primary windings, one of said first primary windings on each of said legs, said first primary windings being dimensioned for full phase capacity, two second primary windings, one of said second primary windings on each of said legs, said second primary windings being dimensioned for half-phase capacity, said two second primary windings being electrically connected, and two secondary windings, one of said secondary windings on each of said legs, said secondary windings being dimensioned for one and a half phase capacity, at least a portion of each of said secondary windings being positioned between said first and said second primary winding on said leg, said secondary windings on said two legs being connected in parallel, all windings extending over substantially the total length of the respective leg.

2. A two-legged transformer as claimed in claim 1, in which said two second primary windings are connected in series.

3. A two-legged transformer as claimed in claim 1, in which said two second primary windings are connected in parallel.

4. A two-legged transformer as claimed in claim 1, in which each of said two secondary windings comprises a major section and a minor section, said major section being positioned between said first and said second primary windings, said minor section being positioned between said leg and said first primary winding.

5. A two-legged transformer as claimed in claim 1, in which said first primary windings are positioned in immediate proximity of said respective leg, and each of said secondary windings is positioned entirely between said first and said second primary winding.

6. A two-legged transformer as claimed in claim 1, in which the percentage short-circuit voltage is approximately the same for all primary windings.

7. A two-legged transformer as claimed in claim 6, in which both said first primary windings are positioned in immediate proximity of said respective legs.

Berg Oct. 23, 1900 Camilli June 11, 1929 

